Heretofore, SIP (Session Initiation Protocol) (refer to RFC3261 “SIP: Session Initiation Protocol”) has been used as a call control protocol when radio communication terminals perform IP packet communications in a radio communication network.
A call control by SIP is performed via an SIP server (a call control server). Specifically, a SIP server receives a call request (INVITE Request) for a destination radio communication terminal (hereinafter, referred to as a destination terminal) from a call originating radio communication terminal (hereinafter, referred to as an originating terminal). After the completion of a login process of the destination terminal, the SIP server transmits the call request from the originating terminal for the destination terminal. Thereafter, a communication path between the destination terminal and the originating terminal is set by the SIP server, and then the call (IP packet communications) is executed by use of the communication path.